1. Field of the Invention
The present invention relates to an optical isolator, and more particularly, to an optical isolator in which isolation is improved and which can be made compact by changing the structure of a photonic crystal.
2. Description of the Related Art
An optical isolator is an irreversible optical element that transmits an optical signal with almost no attenuation in a forward direction and prevents transmission of the optical signal in a reverse direction by dispersing the light. The optical isolator utilizes a phenomenon that an electromagnetic wave propagating parallel to a magnetic field in a dielectric effected by the magnetic field gradually rotates on a polarized surface thereof using the Faraday effect generally seen in a special dielectric in a magnetic filed. When the directions of the magnetic field and the electromagnetic wave are quite opposite, the polarized surface reversely rotates according to the direction of propagation of the electromagnetic wave. When an element such as a polarization plate for passing only an electromagnetic wave on the polarized surface is placed at both sides, an electromagnetic wave in one direction passes well, but an electromagnetic wave in the opposite direction does not pass.
Thus, the optical isolator can prevent the deterioration of a transmission efficiency or damage to optical parts by blocking the reflected optical signal from the optical system.
Referring to FIG. 1, a conventional optical isolator using a polarization mode includes a polarizer 10, a Faraday rotator 13, and an analyzer 15. A light ray L1 propagating in a forward direction has a polarization by the polarizer 10. The polarization of each of light ray L1 having passed through the polarizer 10 is rotated by 45° by the Faraday rotator 13.
The analyzer 15 has a crystal optical axis twisted by 45° in the same direction as the direction in which the light ray L1 is rotated by the Faraday rotator 13 with respect to the crystal optical axis of the polarizer 10. Accordingly, the light ray L1 having passed through the polarizer 10 and the Faraday rotator 13 passes through the analyzer 15. Thus, the light ray L1 propagating in the forward direction passes through the polarizer 10, the Faraday rotator 13, and the analyzer 15.
On the contrary, a light ray L2 propagating in the opposite direction has polarization twisted by 45° by the analyzer. While passing through the Faraday rotator 13, the light ray L2 propagating in the opposite direction is rotated by 45° in a direction opposite to the direction in which the forward directional light ray L1. Thus, the reverse directional light ray L2 having passed through the Faraday rotator 13 has a polarization perpendicular to the crystal optical axis of the polarizer 10. Therefore, the reverse directional light ray L2 is blocked by the polarizer 10 after passing through the Faraday rotator 13.
The polarizer 10 makes an input light ray have one polarization and the Faraday rotator 13 rotates the polarization of the separated light by 45°. The analyzer 15 is arranged in the optical axis direction of 45° with respect to the polarizer 10 and blocks the light having passed the Faraday rotator 13. When the light is input in the opposite direction, the direction of the polarization determined by the analyzer 15 is rotated by (−)45° by the irreversible operation of the Faraday rotator 13 so that the polarization is changed to have a difference of 90° from the polarization of the polarizer 10. Thus, the reverse directional light ray L2 is blocked by the polarizer 10.
The optical isolator is widely used in an optical communication system. When a transmission rate used in the optical communication system increases, performance required for a laser increases as well. Light retro-reflected in part of the optical communication system has a bad effect on the operation of a high performance laser so that the spectrum, line width, or natural noise of the laser is changed. The optical isolator is used to protect a high performance semiconductor laser by restricting the generation of reflective noise generated when light is retro-reflected to the laser. Also, in a dense wavelength division multiplexing method, various optical signals having different wavelengths are simultaneously transmitted through an optical fiber so that transmission capacity is increased, a system cost is reduced, and an efficient network is established. Thus, with a remarkable growth of an ultra-broadband information communication market including an asynchronous digital subscriber network, a need for the optical isolator is drastically increasing over the world. In the dense wavelength division multiplexing method, since the spectrum line width of a light source decreases, maintaining the decreased spectrum line width is important. Also, as integration of optical parts is performed, the structure of the optical isolator needs to be simplified. However, since the conventional optical isolator using the polarizer and the Faraday rotator needs more number of parts and the structure thereof becomes complicated, an optical arrangement is difficult.